1. Field of the Invention
This invention relates to a brushless DC motor having a magnet (i.e.--a field flux generating means) and multiphase coils, and more particularly to a brushless DC motor in which a current is distributed to the multiphase coils so as to be proportional to a reference signal.
2. Description of the Prior Art
In order to control the speed of a brushless DC motor (direct current motor), currents for a plurality of stator coils can be switched and controlled electronically, usually by transistors. A brushless DC motor, which has neither mechanical brushes nor commutators but has contactless electronic commutators such as transistors, has been designed and manufactured to produce a highly reliable motor. In a conventional brushless DC motor, the total armature current is usually controlled by a transistor connected in series with the armature, and the current path to a plurality of stator coils is switched by output transistors operated in then ON or OFF modes.
The output transistors can be used to control the magnitude of current in stator coils when operated in their non-saturating mode. The armature current is controlled indirectly by controlling the base currents of the output transistors. The current flowing through each stator coil varies depending upon h.sub.FE (forward current transfer ratio) of each output transistor. Therefore, it is inevitable that the total current flowing through the stator coils has ripples in the waveform thereof. An externally excited DC motor with a permanent magnet generates a torque which is essentially proportional to the armature current. Therefore, an unbalance or a difference in the h.sub.FE of the output transistors causes a ripple in the generated torque.
U.S. Pat. No. 3,751,676 discloses a brushless DC motor which overcomes the defects of the conventional motor described above. This brushless DC motor has a feedback loop wherein the total current flowing through the stator coils is controlled so as to be proportional to a reference signal irrespective of the unbalance and the variation in the h.sub.FE values of the output transistors. Thus, the torque ripple of the brushless DC motor due to the h.sub.FE unbalance has been significantly reduced. However, a current ripple occurs when the output transistor is saturated, as explained hereinbelow.
A speed controlled brushless DC motor supplies the stator coils with a large current during the starting and accelerating period because a quick response is desired. A alternating counter electromotive voltage, the magnitude of which is proportional to the rotational speed, is generated in each stator coil, and each of the voltage drops across the output transistors decreases according to the increase of the magnitude of each of the counter electromotive voltages. Therefore, the output transistors during each activated period are saturated during a transient period from starting to the desired rotational speed, and the actual supply current to the stator coils has a ripple because the required current corresponding to the reference signal cannot flow and the counter electromotive voltage varies during the activated period.
Furthermore, a spike of current flows to the stator coils by the operation of the feedback loop which activates two output transistors at the same time during a short period of the switching transition. These current ripples produce mechanical vibration and noise during the starting and accelerating period. This is a serious problem for a brushless DC motor used in audio or video equipment.